Manufacture of zinc oxide



Patented Sept. 10, 1935 UNITE sas A'iwttiril HERE MANUFACTURE 0F ZINC OXIDE Earl H. Bunce, Palmerton,

Pa., assigner to The Application March 24, 1930, Serial No. 438,460

1 Claim.

The present invention relates to the manufacture of zinc oxide and has for its object improvements in the method of and apparatus for manu facturing zinc oxide. The invention relates more particularly to the manufacture of zinc oxide from an agglomerated charge of mixed zinciferous material and carbonaceous reducing agent confined Within an externally heated retort.

It has heretofore been proposed to manufacture zinc oxide by subjecting an agglomerated charge of mixed zinciferous material and carbonaceous reducing agent to a reduction operation, conducting the liberated zinc vapor in the form of a stream to an oxidizing environment, and then burning the zinc vapor to form zinc oxide. Such a practice is, for example, disclosed in United States Patent No. 1,678,607. According to said patent, the agglomerated charge is confined Within a vertically disposed and externally heated retort. The gaseous product of the reduction operation, containing the liberated Zinc vapor, is Withdrawn from the retort at or near its upper end, after which the vapor is burned to Zinc oxide. An agglomerated charge is employed that is adapted to break down into fines which tend to seal the bottom of the retort against seepage of outside air into the retort. A similar process is disclosed in United States Patent No. 1,712,132, except that the agglomerated charge does not break down, into fines when its zinc content has been eliminated, but precautions are also taken to prevent the entrance of large amounts of air into the bottom of the retort or the escape of Zinc vapor through the bottom of the retort.

In the reduction of zinc ores, zinc metallurgists have heretofore considered it essential to exclude as far as possible all oxidizing influences. This is particularly true with respect to themanufacture of zinc metal, and is Well illustrated by the usual so-called Belgian retort practice, in which the monolithic retorts are closed at one end, while a condenser is connected with the other end. The presence of air Within the retort tends to form zinc oxide and Zinc dust particles, both of which deleteriously influence the condensation of zinc vapor to zinc metal.

Under carefully controlled operating conditions, the residue discharge end of a zinc reduction chamber may be open to the admission of regulated small amounts of air in the manufacture of zinc metal. A process of this kind is disclosed in copending application Serial No. 180,- 107, led April 1, 1927 which has since issued as United States Patent No. 1,811,910. Such air as is admitted to the reduction step must be insufcient in amount, however, to bring about the formation of an appreciable quantity of Zinc oxide and zinc dust particles in the gaseous and vapor product leaving the chamber, that would be adapted seriously to hinder the subsequent 5 condensation operation. In accordance with the practice disclosed in this copending application, air in relatively small amounts may be admitted into the bottom of the retort to off-set the downward diffusion of zinc vapor; thereby inhibiting l0 the formation of blue powder, or metal in the residue.

My investigations have led to the discovery that relatively large amounts of gas, such as air, may advantageously be admitted into the residue dis- 15 charge end of an externally heated reduction chamber, such as the bottom of a vertically disposed retort, to manufacture zinc oxide when employing an agglomerated charge of mixed zinciferous material and carbonaceous reducing 20 agent.

Such operating conditions lend themselves to the formation of a natural stack or chimney draft. As the reduction operation proceeds to completion, a flow of the resulting evolved gases 25 and liberated zinc vapor may be induced through the retort. The resulting mixture of gases and vapor may be withdrawn from the retort by the aid of the stack draft, and the zinc Vapor is burned to zinc oxide in the presence of air. 'Ihe 30 stack draft effects may, or may not, as desired, be varied by regulating appropriately the amount of air seeping into the residue discharge end of the retort.

Various expedients may be employed in controlling the stack draft effect: Thus, the bottom of the retort may be left open, so that air may readily seep into the same. Any appropriate gas, such as air, nitrogen, Water gas, carbon dioxide; even steam` and the like, may be admitted to the retort, preferably at or near its bottom. The gas-vapor discharge outlet of the retort is preferably located at a level substantially higher than the main reduction Zone of the retort, thereby converting the reduction furnace into a chimneyor stack-like apparatus. Operating conditions maintained Within the retort also influence the stack draft effect. Thus the draft obtained is affected by the porosity of the charge as Well as by the temperature of the charge. The stack draft is also influenced by the rate of flow of the Zinc vapor and retort gases through and from the retort, and by the vaporgas discharge conduit and the apparatus employed to effect the oxidation of the zinc vapor. 55

All these various factors may be properly correlated and regulated in order advantageously to influence the stack draft effects found most suitable in any given case for the production of zinc oxide.

The present invention, therefore, contemplates the method of manufacturing zinc oxide from an agglomerated charge of mixed zinciferous material and carbonaceous reducing agent in which the agglomerates are confined within an externally heated retort, inducing a flow of evolved gases and liberated zinc vapor through the retort by controlled stack draft, withdrawing the resulting mixture of gases and vapor from the retort by the aid of said stack draft, and burning the zinc vapor in the presence of air to form zinc oxide.

In the case of the use of a vertically disposed and externally heated retort, regulated amounts of gas may be admitted into the lower end of the retort. Thus, air, nitrogen, water gas, carbon dioxide, steam and the like, may be admitted into the vertically disposed retort at or near its lower end by induced or forced draft, which gas aids in giving an upward movement to the gaseousvapor product of the reduction action. This is desirable because such a movement tends to prevent the escape of zinc vapor through the bottom of the retort.

The admission of air in relatively large amounts at the bottom of the retort is particularly desirable in the manufacture of zinc oxide, since the dilution of the liberated Zinc vapor by the nitrogen of the air and by the carbon monoxide originating from the oxygen of the air is favorable for the production of fine particle size zinc oxide. If a concentrated stream of zinc vapor is con-- ducted into the open atmosphere, for example, it will burn in a characteristic lazy flame to form zinc oxide particles that are relatively large. If, -on the other hand, the stream of zinc vapor is heavily diluted with gases inert to the vapor, the vapor will burn in the open air to form relatively small particles of zinc oxide. The presence of large quantities of inert gases with the newly formed zinc oxide particles, moreover, tends to keep them Yfar apart and thus hinders particle growth due to the sublimation of zinc oxide from the small particles onto the larger particles. The degree of growth of the particles and thus their size can be controlled by regulating the degree of the dilution of the zinc vapor issuing from the retort and the temperature of the exhaust gases and air in which the newly formed zinc oxide is in suspension.

The introduction of large amounts of air into ,the retort materially increases its reducing efficiency. Carbon `monoxide formed vby the reaction of the atmospheric oxygen with the carbon in the charge is a reducing gas that is an active factor in reducing the zinc compounds, such as zinc oxide, present in the charge. The increase in the volume of gases going up the retort also aids in carrying heat from the highly heated retort walls, externally applied, to the core of the charge.

The reaction of reduction within the retort is also materially aided by the heat of combustion produced by the combination of the carbon in the charge with atmospheric oxygen of the air to form carbon monoxide. In other words, considera'ble heat is directly supplied within the retort to the reduction reaction. This heat serves to supplement that externally applied to the retort. Due to the extra heat thus made available for the reduction operation, an increased amount of Zinc oxide present in the charge may be treated in the retort per unit of time. Moreover, the heat thus additionally supplied takes place in the lower section of the retort where it is desirable to maintain the highest temperatures.

It is believed that the present invention may best be understood by a consideration of the accompanying drawing, taken in conjunction with the following description in which is shown an apparatus in elevation and section adapted for the practice of the invention:

The apparatus shown comprises a furnace structure It resting on concrete foundations II. The furnace structure consists of a bottom I2, side walls I3, and an arch roof I4 constructed of suitable heat-resistant material, such as refractory brick. A metallic casing l5 completely surrounds the side walls and the top of the furnace structure. The space between the arch roof and the metallic casing on the top of the furnace structure is filled with a suitable heat-insulating material i6, such as diatomaceous earth or the like.

The heat refractory linings of the furnace structure define a heating chamber I'I. A suitable port, or ports, I8, extending completely through the side walls of the furnace structure, 4are advantageously spaced throughout the height of the heating chamber at intervals adapted to give optimum heating conditions. These ports are adapted for the introduction of air and suitable fuel to provide the necessary hot combustion gases. Thus, gas, oil, or pulverized coal may be introduced through these ports. An exit I9 is provided through one of the side walls at or near the upper end of the heating chamber for the withdrawal of heating gases. This exit preferably connects with a stack or chimney (not shown) Spaced openings 20 are provided at desirable intervals throughout the height of the furnace structure, leading into the heating chamber in order to take pyrometric readings.

A vertically disposed reduction retort 2i extends completely through -and centrally of the heating chamber and its surrounding furnace structure. The retort is supported by the furnace structure only at its lower end, where, for example, it may rest on the bottom of the furnace structure. This permits free expansion and contraction of the retort independently of the fur- I nace structure. A sleeve member 22, which may consist of an appropriately shaped casting, is attached to the underside of the furnace structure, its passageway being in substantial alignment with that of the retort above. form 23, moving about a vertical axis, is located directly below the lower end of the retort, and is adapted to effect a regulated withdrawal of spent residues 2li from the retort. This platform may be made to rotate constantly, if residues are to be removed substantially continuously. On the other hand, if the reduction process is to be operated intermittently, say in batches, the platform may be turned whenever it is desired to remove spent residues.

The upper end of the retort extends a convenient dist-ance above the upper end of the furnace structure. It preferably constitutes a substantial extension or prolongation 25, that may be employed as an eliminator. process of reducing zinciferous material outlined in my copending application, Serial No. 244,401, filed January 4, 1928 which has since issued as United States Patent No. 1,749,126, zinc vapor contaminated with objectionable amounts of lead According to the F A revolving platl and the like may be continuously conducted through a hot body of agglomerated charge materials about to be subjected to the reduction operation. This body of hot agglomerates serves to filter out and retain the objectionable lead, permitting a substantially lead-free zinc vapor to pass on for subsequent treatment. Provision for the practice of that process is advantageously made in the apparatus of the present invention, so that a relatively pure zinc oxide pigment may be made; at least when desired.

The eliminator is defined by an eliminator structure 26, consisting of an outer metallic casing 2'! which may be varied in size. The space between the metallic casing and the eliminator is filled with a suitable heat-insulating material 23, such as .dust coal. The amount of heat-insulating material used is regulated so that the dissipation of heat may be appropriately controlled. In order to eliminate lead from the zinc vapor, it is necessary to keep hot agglomerated charge materials 2 9 confined within the eliminator at a temperature adapted to takeout the lead, while avoiding any appreciable condensation of zinc vapor.

A suitable door, or doors, 3i] is provided at or near the lower .end of the eliminator structure for the withdr-awal of heat-insulating material, if that should become desirable. The upper end of the eliminator structure is preferably left open for the introduction and withdrawal of heat-insulating material. Since the metatllic casing 2l may be varied in size, it follows that the thickness of the layer of heat-insulating material 23 may be likewise varied.

An off-take pipe or conduit 3! extends through the eliminator structure at or near its upper end, which is adapted for the withdrawal of gases and vapor. A metallic charging device 3?; rests within the upper end of the eliminator, and extends a suitable distance below the opening of the offtake pipe so that charge materials may be kept out of the same. .A removable cap 33 rests over the upper end of the charging device. it may be removed from time to time for the introduction of fresh charge materials. If the reduction process is to be operated continuously, this cap is preferably removed and the charging device is appropriately connected to a source adapted to feed charge materials into the apparatus as rapidly as desired.

A zinc vapor treatment device 34 connects with the gas off-take pipe. This device rests on a metallic plate 35, located on the top of spaced I- beams 36, which are in turn supported by the furnace structure below. The lower end of the vapor treatment device consists of a foundation of ordinary red brick tl, above which rest suitable amounts of heat-refractory brick 3b. Since the apparatus is intended for the manufacture of zinc oxide, the vapor treatment device is designed for that purpose. It consists of outer walls 39 constructed of suitable heat-resistant brick. These walls define an inner chamber @il in which the zinc vapor coming from the retort is burned to zinc oxide. A baille wall M extends laterally and part way across the chamber. This baiile wall is adapted to deflect zinc vapor coming from the off-take pipe. A suitable number of air inlet openings i2 are provided in the side walls of the chamber below the bafiie wall for the introduction of atmospheric air. These air inlets are sufcient in number and size to permit adequate ingress of air to burn all of the zinc vapor. In fact, an excess Iof air may be permitted to enter the combustion chamber in this manner.

A clean-out opening 43 is located in the zinc vapor treatment device in substantial alignment with the off -take pipe. This opening is preferably closed when the apparatus is in operation. Simiv lar clean-out openings 44 and 45 are provided in 5 the side walls of the device for the purpose of giving access to the inner combustion chamber below and above the baffle wall, respectively. The roof of the inner chamber consists of tile plates 4S having an appropriate opening 4l'. 'I'he size 10 of the hole may be regulated by means of a slide damper. This opening leads into a chimney-like extension 48 which is cylindrical in shape. Its side wall d@ is constructed of suitable heatrefractory brick. A peep-hole 5B extends through 15 the wall. The upper end of this extension connects with a pipe 5l leading to collecting chambers where the zinc oxide particles are 'separated from the gases. This pipe has a suitable connection with a suction fan (not shown) adapted to transport the newly formed zinc oxide particles to the collecting chamber.

The process of the present invention may advantageously be practiced in the above described apparatus as follows:

The cap 33 is removed and an appropriately agglomerated charge of mixed zinciferous material and carbonaceous reducing agent 29 is introduced into the charging device 32. A sufficient amount of agglomerates are thus introduced until 30 the retort and the eliminator are filled to a level within the charging device 32 at or above the off -take 3 l.

Air and fuel, such as oil, in regulated amounts are introduced through the ports I8. The com- 35 bustion gases circulate within the heating chamber il, and ultimately find their way through the exit i9 and stack (not shown). Pyrometric readings are taken through the openings 20 from time to time to observe temperature conditions within the heating chamber. Suitable changes are made in the amounts of air and fuel supplied through the ports in order to conduct the reduction operation at appropriate temperatures.

As the heat is conducted through the retort walls, the agglomerates confined therein are gradually raised to their reduction temperature. The reduction of the Zinc compounds present in in the charge results in the liberation of zinc vapor and the evolution of retort gases. These tend to rise in the form of a mixture by a natural upward draft. As pointed out above, the apparatus is designed to make such a stack or chimney draft possible. Controlled amounts of air are permitted to seep into the lower end of the retort, the air passing through the voids or spaces provided between contacting spent residues 24.

The mixture of gases and vapor gradually finds its Way up through the hot agglomerates confined within the eliminator 25. Dissipation of heat from the eliminator is carefully controlled, so that optimum temperature conditions may prevail selectively to retain lead present in the zinc vapor, while substantially inhibiting the condensation of zinc vapor, thus leaving a substantially lead-free zinc vapor to pass through the offtake pipe 3l. As reduction of the charge materials proceeds to completion, the revolving platform 23 is set in motion and spent residues are withdrawn. Fresh charge materials are at the same time introduced into the charging device 32.

The Zinc vapor and its accompanying gases ultimately find their way into the vapor treatment device 34. At the same time, surrounding atmosphere rushes into the air inlets 42, where it mingles in the chamber 40 with the incoming zinc vapor. The hot zinc vapor promptly combines with the oxygen of the air to form zinc oxide. The newly formed zinc oxide particles, together with accompanying gases, finds its way through the outlet hole 41 and into the extension 48; from which the mixture is ultimately withdrawn in part at least by suction to the collecting chamber, where the zinc oxide particles arefinally separated from the gases.

When reduction of the zinc compounds present in the charge takes place, retort gases are evolved and zinc vapor is liberated. Without stack draft control, pressures are set up within the retort,

' and, under these operating conditions, both gases and vapor tend to escape from the lower end of the retort; as Well as from the upper end of the retort. Any zinc vapor that would escape from the lower end of the retort would of course be permanently lost. The present invention, however, contemplates the provision of a natural upward draft of the hot gases and zinc vapor that prevents the escape of vapor from the lower end of the retort; which draft may advantageously be augmented by the applied or forced admission of suitable gases into the retort.

I claim:

In the manufacture of pigment zinc oxide the improvement which comprises heating a charge of mixed and agglornaterated zinciferous and carbonaceous material in an elongated vertical reduction chamber to produce and volatilize zinc, withdrawing the resulting zinc Vapor from the top of the reduction chamber, oxidizing the zinc vapor to zinc oxide in a current of air, introducing large amounts of air into the base of the reduction chamber to prevent the downward diffusion of zinc vapor in the reduction chamber and to dilute the zinc vapor in the reduction chamber with gaseous products of combustion resulting from the reaction of the air introduced into the base of the reduction chamber with the carbonaceous material in the reduction chamber, and varying the amount of air introduced into the base of the reduction chamber to vary the concentration of zinc vapor in the gaseous products of 20 combustion withdrawn from the top of the reduction chamber, and thereby control the particle size of the zinc oxide produced. 

